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NASA Selects Final Four Site Candidates For Asteroid Sample Return

Pictured are the four candidate sample collection sites on asteroid Bennu selected by NASA’s OSIRIS-REx mission. Site Nightingale (top left) is located in Bennu’s northern hemisphere. Sites Kingfisher (top right) and Osprey (bottom left) are located in Bennu’s equatorial region. Site Sandpiper (bottom right) is located in Bennu’s southern hemisphere. In December, one of these sites will be chosen for the mission’s touchdown event. Image Credit: NASA/University of Arizona

August 12, 2019 – After months grappling with the rugged reality of asteroid Bennu’s surface, the team leading NASA’s first asteroid sample return mission has selected four potential sites for the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft to “tag” its cosmic dance partner.

Since its arrival in December 2018, the OSIRIS-REx spacecraft has
mapped the entire asteroid in order to identify the safest and most
accessible spots for the spacecraft to collect a sample. These four
sites now will be studied in further detail in order to select the final
two sites – a primary and backup – in December.

The team originally had planned to choose the final two sites by this
point in the mission. Initial analysis of Earth-based observations
suggested the asteroid’s surface likely contains large “ponds” of
fine-grain material. The spacecraft’s earliest images, however, revealed
Bennu has an especially rocky terrain. Since then, the asteroid’s
boulder-filled topography has created a challenge for the team to
identify safe areas containing sampleable material, which must be fine
enough – less than 1 inch (2.5 cm) diameter – for the spacecraft’s
sampling mechanism to ingest it.

“We knew that Bennu would surprise us, so we came prepared for
whatever we might find,” said Dante Lauretta, OSIRIS-REx principal
investigator at the University of Arizona, Tucson. “As with any mission
of exploration, dealing with the unknown requires flexibility, resources
and ingenuity. The OSIRIS-REx team has demonstrated these essential
traits for overcoming the unexpected throughout the Bennu encounter.”

The original mission schedule intentionally included more than 300
days of extra time during asteroid operations to address such unexpected
challenges. In a demonstration of its flexibility and ingenuity in
response to Bennu’s surprises, the mission team is adapting its site
selection process. Instead of down-selecting to the final two sites this
summer, the mission will spend an additional four months studying the
four candidate sites in detail, with a particular focus on identifying
regions of fine-grain, sampleable material from upcoming,
high-resolution observations of each site. The boulder maps that citizen
science counters helped create through observations earlier this year
were used as one of many pieces of data considered when assessing each
site’s safety. The data collected will be key to selecting the final two
sites best suited for sample collection.

In order to further adapt to Bennu’s ruggedness, the OSIRIS-REx team
has made other adjustments to its sample site identification process.
The original mission plan envisioned a sample site with a radius of 82
feet (25 m). Boulder-free sites of that size don’t exist on Bennu, so
the team has instead identified sites ranging from 16 to 33 feet (5 to
10 m) in radius. In order for the spacecraft to accurately target a
smaller site, the team reassessed the spacecraft’s operational
capabilities to maximize its performance. The mission also has tightened
its navigation requirements to guide the spacecraft to the asteroid’s
surface, and developed a new sampling technique called “Bullseye TAG,”
which uses images of the asteroid surface to navigate the spacecraft all
the way to the actual surface with high accuracy. The mission’s
performance so far has demonstrated the new standards are within its
capabilities.

“Although OSIRIS-REx was designed to collect a sample from an
asteroid with a beach-like area, the extraordinary in-flight performance
to date demonstrates that we will be able to meet the challenge that
the rugged surface of Bennu presents,” said Rich Burns, OSIRIS-REx
project manager at NASA’s Goddard Space Flight Center in Greenbelt,
Maryland. “That extraordinary performance encompasses not only the
spacecraft and instruments, but also the team who continues to meet
every challenge that Bennu throws at us.”

The four candidate sample sites on Bennu are designated Nightingale, Kingfisher, Osprey, and Sandpiper – all birds native to Egypt. The naming theme complements the mission’s two other naming conventions – Egyptian deities (the asteroid and spacecraft) and mythological birds (surface features on Bennu).

The four sites are diverse in both geographic location and geological
features. While the amount of sampleable material in each site has yet
to be determined, all four sites have been evaluated thoroughly to
ensure the spacecraft’s safety as it descends to, touches and collects a
sample from the asteroid’s surface.

Nightingale is the northern-most site, situated at 56 degrees north
latitude on Bennu. There are multiple possible sampling regions in this
site, which is set in a small crater encompassed by a larger crater 459
feet (140 m) in diameter. The site contains mostly fine-grain, dark
material and has the lowest albedo, or reflection, and surface
temperature of the four sites.

Kingfisher is located in a small crater near Bennu’s equator at 11
degrees north latitude. The crater has a diameter of 26 feet (8 m) and
is surrounded by boulders, although the site itself is free of large
rocks. Among the four sites, Kingfisher has the strongest spectral
signature for hydrated minerals.

Osprey is set in a small crater, 66 feet (20 m) in diameter, which is
also located in Bennu’s equatorial region at 11 degrees north latitude.
There are several possible sampling regions within the site. The
diversity of rock types in the surrounding area suggests that the
regolith within Osprey may also be diverse. Osprey has the strongest
spectral signature of carbon-rich material among the four sites.

Sandpiper is located in Bennu’s southern hemisphere, at 47 degrees
south latitude. The site is in a relatively flat area on the wall of a
large crater 207 ft (63 m) in diameter. Hydrated minerals are also
present, which indicates that Sandpiper may contain unmodified
water-rich material.

This fall, OSIRIS-REx will begin detailed analyses of the four
candidate sites during the mission’s reconnaissance phase. During the
first stage of this phase, the spacecraft will execute high passes over
each of the four sites from a distance of 0.8 miles (1.29 km) to confirm
they are safe and contain sampleable material. Closeup imaging also
will map the features and landmarks required for the spacecraft’s
autonomous navigation to the asteroid’s surface. The team will use the
data from these passes to select the final primary and backup sample
collection sites in December.

The second and third stages of reconnaissance will begin in early 2020 when the spacecraft will perform passes over the final two sites at lower altitudes and take even higher resolution observations of the surface to identify features, such as groupings of rocks that will be used to navigate to the surface for sample collection. OSIRIS-REx sample collection is scheduled for the latter half of 2020, and the spacecraft will return the asteroid samples to Earth on September 24, 2023.

Goddard provides overall mission management, systems engineering, and
safety and mission assurance for OSIRIS-REx. Dante Lauretta of the
University of Arizona, Tucson, is the principal investigator, and the
University of Arizona leads the science team and the mission’s science
observation planning and data processing. Lockheed Martin Space in
Denver built the spacecraft and is providing flight operations. Goddard
and KinetX Aerospace are responsible for navigating the spacecraft.
OSIRIS-REx is the third mission in NASA’s New Frontiers Program, which
is managed by NASA’s Marshall Space Flight Center in Huntsville,
Alabama, for the agency’s Science Mission Directorate in Washington.